Wireless telecommunication systems provide information services traditionally provided by land-line or copper wire systems. Examples of wireless communications applications include Advanced Mobile Phone Service (AMPS) analog cellular service and AMPS-D digital cellular service in North America, and Group Speciale Mobile (GSM) cellular service in Europe.
Although the particular application may vary, the components of a wireless communication system are generally similar. For example, a wireless communication system usually includes a radio terminal or mobile station, a radio base station, a switch or network control device, often referred to as a mobile telephone switching office (MTSO), and a network to which the wireless communications system provides access, such as the Public Switched Telephone Network (PSTN).
The various wireless communication applications use different modulation techniques for transmitting information to more efficiently utilize the limited available frequency spectrum. For example, frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA) modulation techniques are used to build high-capacity multiple access systems. Telecommunication systems designed to communicate with many mobile stations occupying a common radio spectrum are referred to as multiple access systems.
For example, in an FDMA analog cellular system, such as an AMPS analog cellular radio system, the available frequency spectrum is divided into a large number of radio channels, e.g., pairs of transmit and receive carrier frequencies, each of which corresponds to a message transmission channel. The bandwidth of each transmit and receive frequency channel is narrowband, generally 25-30 kHz. Thus, the FDMA system permits information to be transmitted in a bandwidth comparable to the bandwidth of the transmitted information, such as a voice signal. The cellular service area in the FDMA system is generally divided into a plurality of cells, each cell having a set of frequency channels selected so as to minimize co-channel interference between cells.
Frequency division is often combined with time division so that transmission circuits are trunked in both the frequency and time domain, e.g., a FD/TDMA system. In a digital FD/TDMA (commonly referred to as TDMA) cellular system, a narrowband frequency channel is reformatted as a digital transmission path which is divided into a number of time slots. The data signals from different calls are interleaved into assigned time slots and sent out with a correspondingly higher bit rate, the time slot assigned to each mobile station being periodically repeated. Although the TDMA bandwidth may be somewhat larger than the FDMA bandwidth, a bandwidth of approximately 30 kHz is generally used for AMPS-D digital TDMA cellular systems.
A very different approach to cellular multiple access modulation is CDMA. CDMA is a spread spectrum technique for transmitting information over a wireless communication system in which the bandwidth occupied by the transmitted signal is significantly greater than the bandwidth required by the baseband information signal (e.g., the voice signal). Thus, CDMA modulation spectrally spreads a narrowband information signal over a broad bandwidth by multiplex modulation, using a codeword to identify various signals sharing the same frequency channel. Recognition of the transmitted signal takes place by selecting the spectrally-coded signals using the appropriate codeword. In contrast to the narrowband channels of approximately 30 kHz used in FDMA and TDMA modulation techniques, a CDMA system generally employs a bandwidth of approximately 1.25 MHz or greater.
Regardless of the modulation technique used in a cellular telecommunication system, when a mobile station is in communication with its base station, for example to provide telephone service between a mobile station and a calling party, the cellular system must maintain uninterrupted service for the call despite movement of the mobile station through the cellular system. For example, in an analog cellular system, when the mobile station transitions from one cell to another cell, the mobile station must change frequencies because each cell supports a different set of frequencies. The process by which a cellular telecommunications system enables a mobile station to maintain an established connection when moving through cells of a cellular system is referred to as "handoff," and is generally controlled by the MTSO.
In a conventional analog cellular system, a handoff is triggered when the base station currently providing the link between the mobile station and the MTSO detects that the received signal strength from the mobile station has dropped below a predetermined level. The low signal strength from the mobile station usually indicates that the mobile station is approaching the boundary of the cell. When the received signal strength is below the predetermined value, the base station requests the MTSO determine whether another base station, e.g., a neighboring base station, is receiving a stronger signal from the mobile station than the current base station.
In response to the request from the current base station, the MTSO sends a message to the appropriate neighboring base stations to measure the received signal strength from the mobile station. The neighboring base stations, using a scanning receiver, monitor the frequency channel of the mobile station and measure the received signal strength, if possible. The measurements made by the neighboring stations are reported to the MTSO. If one of the neighboring base stations receives the mobile station signal above a predetermined level, then the MTSO directs a handoff of the mobile station from its current base station to a new base station in an adjoining cell. In particular, the MTSO informs the mobile station of a new frequency to be used with the new base station, while the MTSO also switches the call from the current base station to the new base station. If the handoff is unsuccessful, however, the call will be lost, e.g., terminated. This type of handoff is often referred to as a system-assisted handoff because the cellular system controls the detection of the need for, and the execution of, the handoff.
Another type of handoff is referred to as a mobile-assisted handoff (MAHO). For example, in a digital CDMA cellular system, each base station transmits a CDMA pilot signal on a common frequency, each pilot signal being differentiated by its phase offset compared to other pilot signals. A mobile station located in a digital CDMA cellular system regularly monitors the pilot signal strength received from the various pilot signals of neighboring base stations. The mobile station detects when the received signal strength of a pilot signal from its current base station has dropped below a predetermined level and the received signal strength of a neighboring base station pilot signal exceeds a predetermined level. The mobile station transmits these signal strength measurements to the MTSO via the base station with which the mobile station is in communication. The MTSO directs a handoff from one base station to another base station based on the signal strength measurements made by the mobile station.
A conventional narrowband analog cellular system, such as an AMPS FDMA cellular system, cannot support MAHO because in the analog system there is no pilot signal, the mobile station does not take measurements of the signals transmitted by the analog base station, and the handoff is controlled by the base stations and MTSO. Moreover, a 30 kHz analog cell base station cannot transmit a 1.25 MHz CDMA pilot signal.
Similar to the CDMA system MAHO, in a digital TDMA cellular system, each base station can transmit a unique 30 kHz beacon signal that is received and measured by the mobile station and reported to the MTSO. Based on the frequency of the beacon signal, the MTSO can identify the cell site associated with each beacon signal. When the received beacon signal strength drops below a predetermined value, then the mobile station reports the measurement to the MTSO, via a base station, and the MTSO can direct a handoff of the mobile station to another base station, either analog or digital TDMA, associated with a sufficiently strong beacon signal.
A TDMA to analog handoff is possible because both the TDMA system and the analog system are narrowband systems using 30 kHz frequency channels. Thus, a 30 kHz analog cell base station can support a TDMA MAHO handoff using a 30 kHz TDMA pilot signal. The TDMA MAHO has problems with false handoffs, however, because a mobile station can receive a 30 kHz signal that is not a beacon signal but rather is a communication signal from another mobile station. For example, a mobile station at a high elevation may transmit a 30 kHz signal on the same frequency as a particular beacon signal that is mistakenly detected as a beacon signal by another mobile station at a lower elevation, thus causing an unwarranted handoff and possibly a lost call.
In CDMA cellular telecommunication systems, a handoff is usually accomplished via a "soft handoff" from one base station to another base station. In a soft handoff, the mobile station is in communication with more than one base station simultaneously, and thus the mobile station performs a "make before break" transition from one base station to another base station. The soft handoff is possible because in CDMA cellular telecommunication systems, numerous mobile stations communicate with each base station on the same frequency channel, each mobile station having a unique spreading code for distinguishing the information signals broadcast by the numerous mobile stations. Thus, when a mobile station moves from one CDMA cell to another CDMA cell, the same frequency is used in each CDMA cell and the unique spreading code identifies the mobile station to the new base station.
In contrast to the soft handoff used in CDMA cellular systems, narrowband frequency modulation systems, such as FDMA and TDMA systems, employ a "hard handoff." The hard handoff, which is a "break before make" connection, is necessary in narrowband cellular systems because each mobile station is communicating with a base station on a particular narrowband frequency channel. The available frequency channels in adjoining cells differ, and thus when a mobile station moves from one cell to another cell, a new frequency channel must be used.
The advantage of employing a narrowband modulation scheme, such as FDMA, would be defeated if such a system utilized a soft handoff. For example, a narrowband FDMA cellular telecommunication system using a soft handoff would require that the mobile station simultaneously communicate with at least two base stations in adjoining cells on either the same or different frequencies. If the mobile station communicated on the same frequency to two adjoining base stations, co-channel interference would result from two base stations broadcasting on the same frequency to the mobile station, precisely the type of interference the narrowband system was designed to avoid. Alternatively, requiring the mobile station transmit its communication signal to at least two base stations in adjoining cells on two separate frequencies simultaneously is not possible because such simultaneous communication capability is not possessed by conventional mobile stations.
As spread spectrum modulation techniques, such as CDMA, are implemented within existing cellular telecommunications systems, compatibility issues arise regarding the integration of CDMA cell sites into existing analog cellular telecommunications systems. The commercial success of a cellular service provider is dependent in part on the provider's ability to provide seamless integration of new CDMA cell sites into existing analog systems, and in particular, the ability to have unnoticeable handoffs as a mobile station transitions from the CDMA portion of the system into the analog portion of the system.
One problem with integrating CDMA cells into existing analog cellular systems is the inability of conventional mobile stations to support CDMA and analog communications simultaneously. Conventional mobile stations provide a dual mode capability for generating and receiving spread spectrum and narrowband signals. The mobile stations, however, can operate in only one mode at a time. Therefore, while a mobile station is communicating on the cellular system via a CDMA channel, e.g., a 1.25 MHz channel, it is not possible for the mobile station to simultaneously communicate via a narrowband channel of the system, e.g., a 30 kHz channel.
Another problem is that a narrowband base station cannot receive a spread spectrum CDMA signal to measure the received signal strength necessary to perform a system-assisted handoff, as the CDMA signal is spread over a bandwidth that is larger than the narrowband channel which the narrowband base station is designed to receive. Also, a narrowband base station transmits a narrowband signal, e.g., a 30 kHz signal, and thus cannot provide a CDMA pilot signal to be received and measured by the mobile station to facilitate a MAHO to an analog base station. The handoff of a mobile station from a CDMA cell site to an analog cell site represents one of the more significant problems with integrating CDMA cell sites into existing cellular systems.
Current approaches to the problem of handoff of a mobile station from a CDMA portion of a cellular telecommunications network to an analog portion of the telecommunications network are inefficient and affect performance. For example, an additional analog cell can be placed in the CDMA cell for an internal handoff of the mobile station prior to handoff of the mobile station to the existing analog system.
Under this approach, when a handoff of a mobile station from the CDMA portion of the system to the analog portion is necessary, a handoff is first performed from the CDMA base station to the additional analog base station in the same cell, i.e., the CDMA cell is actually a digital/analog cell, capable of supporting both types of modulation. Assuming, however, that the mobile station is transitioning beyond the boundary of the digital/analog cell to an analog cell, another handoff is required to an analog base station of the existing analog system. Thus, two hard handoffs are required for the transition of a mobile station across only one boundary, whereas only one handoff would be desirable.
In addition to requiring an unnecessary handoff within the CDMA cell, the above approach presents other problems. For example, the handoff from the CDMA base station to the analog base station in the same cell is a "blind handoff." As described above, the bandwidths of spread spectrum and narrowband frequency channels are incompatible, as are the types of modulation techniques. Thus, the CDMA to analog handoff in the same cell site is directed without the benefit of knowing that the target analog base station is indeed the best target base station, or with what strength the communication signals from the mobile station will actually be received by the target analog base station. As a result of the lack of information on the suitability of the handoff, it is possible that the handoff might not be properly executed, resulting in a lost call, e.g., termination of the call.
Another problem with this approach to CDMA to analog handoffs is the necessity of reducing the available coverage area of the CDMA cell. A benefit of a CDMA cell, as well as digital cells generally, is that the cellular service area provided by the CDMA cell, often referred to as its "footprint," is larger than the footprint of conventional analog cells. However, in order to perform a CDMA to analog handoff within the CDMA cell that has a significant chance of success, the footprint of the CDMA cell must be reduced so that there is sufficiently strong analog coverage when the CDMA to analog handoff in the same cell site actually occurs.
A further problem with this approach to CDMA to analog handoff is unwarranted handoffs to the analog portion of the cellular telecommunications system. For example, using the MAHO scheme that is implemented in a CDMA cellular system, a handoff is directed when the received signal strength of the pilot signal from a base station with which the mobile station is in communication drops below a predetermined level. Thus, a mobile station may enter an area within the coverage area of a CDMA cell but for some reason, the CDMA pilot signals received by the mobile station are attenuated. For example, the mobile station could enter an underground parking garage. If the pilot signals are below the predetermined value, then the MTSO will direct a handoff to the analog portion of the cellular system. The handoff, however, generally will not solve the problem of the mobile station's poor reception in the underground parking garage, and thus prematurely takes the mobile station off the CDMA portion of the cellular system. In conventional cellular systems having analog and CDMA portions, there is generally no provision for handoff from an analog portion of the system back to the CDMA portion of the system. Therefore, this approach to CDMA to analog handoff allows unwarranted and unnecessary handoffs to the analog portion of a cellular telecommunications system when the actual preference of the cellular system is to keep the mobile station on the digital portion of the system as long as possible.
Another approach to CDMA to analog handoff is a direct handoff from the CDMA cell site to the desired analog cell site, thus avoiding an interim analog handoff. This approach, however, has some of the same problems as the above CDMA to analog handoff method. In particular, the direct handoff approach is a blind handoff to the analog cell site due to the lack of information available to the mobile station about the analog base station while the mobile station is in communication with the CDMA cell site.
Therefore, a need exists for a method of directing a handoff of a mobile station from a spread spectrum portion of a cellular system to an analog portion of the cellular system which minimizes additional equipment costs, avoids unnecessary handoffs to the analog portion of the system, and ensures continuation of an existing call in the analog portion of the system upon completion of the handoff.